Recent developments in microfabrication techniques have permitted the integration of microminiature tools for biochemical analysis within a tiny device. Complete chemical processing systems, e.g., reaction chambers, separation capillaries and their associated electrode reservoirs, as well as certain types of detectors, can be consolidated on a microchip of, e.g., a glass or fused silica. Such "laboratories-on-a-chip," in principle, permit effective utilization and manipulation of minute quantities of material. After the intended procedures have been conducted, the processed compounds are available on the chip in a spatially concentrated form that is suitable for performing further analytical operations. As the sample components are in volumes on the order of nanoliters, subsequent operations should preferably be carried out on the same device. (See, e.g, Effenhauser et al., Anal. Chem. 67:2284-2287, 1995.) This constraint, however, permits less than efficient utilization of certain powerful analytical instruments, such as a mass spectrometer.